When loading & upgrading the chart this way:
chart = await client.get_chart(chart_ref='path/to/helm/chart')
revision = await client.install_or_upgrade_release(
'release_name',
chart,
values,
namespace='namespace'
)
I get
pyhelm3.errors.Error: Error: invalid chart URL format: 'path\to\helm\chart'
Problem are the apostrophes in the final shell_formatted_command-string (see command.py, line 185), which results from the conversion chart_ref-string --> WindowsPath --> str.
When adding
shell_formatted_command = shell_formatted_command.replace('\'', '')
after line 183 in command.py everything works as expected.

Maybe you can add this fix (or a similar one) to the library !?